One of the most common and economical methods of making ceramic bodies includes a step of shaping ceramic powder dispersions containing a fugitive binder. The binder serves to hold the shaped powder together at the molding temperature, and usually also at room temperature if that is different, in a coherent but usually fragile type of body known as a green body or greenware. (A ceramic body is often called "green" or "greenware" even at later stages of processing if it is still substantially more fragile than after final densification.) A green body is converted to a directly useful ceramic body by controlled heating, which both expels the binder by volatilization, combustion, or other chemicals of ceramic powder to bond to one another.
The process of expelling the binder is denoted as "dewaxing" (a customary term in the art, derived from the fact that binders are often waxes, but applicable even if the binder has some other chemical nature.) Any process of converting the ceramic content from separated or only weakly self-bonded powder to a strongly coherent body is denoted generally herein as "densification". This term thus embraces the distinct types of processes known as sintering (used herein to mean heating, without application of external pressure, at a sufficiently high temperature to cause the ceramic particles to bond together by any process that reduced the porosity of the body formed from the powder), uni-axial pressing, isostatic pressing, etc.
Dewaxing and densification sometimes proceed simultaneously, but more often there is little or no densification until dewaxing is complete, and almost always densification is begun or continued by heating dewaxed bodies at a higher temperature after dewaxing is complete.
Depending on the fraction of the dispersion which is ceramic, the characteristics of the the ceramic powder, and the times and temperatures used for densification, final products which are either impervious or have controlled porosity can be produced. Especially when impervious final products are desired, it is advantageous to have as high a ceramic powder content in the dispersion as is practicable, because the time required for densification is thereby reduced, as is the shrinkage of the body during densification. As a generalization, small particle size ceramic powders are preferred, because fine powders tend to densify more readily.
Despite the recognized advantages of both small particle size and high loadings of powders in dispersions, there is a practical limit on increasing both of these characteristics of dispersions, because small particle sizes and high loadings produce dispersions with high viscosity. A viscosity higher than 2000 poises (hereinafter abbreviated as "p") as measured by a capillary rheometer is considered impractical for commercial injection molding of complex shapes, and a viscosity range from 400-1000 p is generally preferred for this shaping technique. Still lower viscosity is preferred for slip casting.
It is customary in the art to measure the specific surface area of ceramic powders as a rough indication of the powder particle size. Very fine powders must have a high surface area as a consequence of the mathematical fact that the surface to volume ratio of any specific solid shape, such as a sphere, increases as the size of the solid decreases. However, it is well known in the art that part of the specific surface area of actual powders is not connected directly with the size of the powders but instead is due to internal porosity or to surface concavities. That part of the powder surface area due to the presence of concavities, pores, or the like is denoted herein as "internal surface", while the remainder of the powder surface area, that part that would remain if all such hollows were filled to give each powder particle a smooth flat or convex surface coinciding with the outermost portions of its actually surface, is denoted herein as "external surface."
Internal surface and its associated volume in ceramic molding powders is usually wholly undesirable, without any compensating benefits. Voids in the powder may result in pores in the final product, which are usually undesirable, or in unexpected and random mechanical failures, which are always undesirable. Internal surfaces adsorb surface active agents that may be added to the powders or dispersions to decrease viscosity. These surface active agents are usually expensive, so that the need to use more of them as a result of adsorption on internal surface can be a serious economic impediment to the use of powders with a high internal surface.
Another problem is that tortuously shaped internal cavities can trap binder if the only outlet(s) from a cavity become blocked by local densification before all the binder is expelled. Such trapped binder can result in residual carbon in the ceramic and/or result in mechanical failure of the bodies. Sometimes the hazard of trapping the binder can be avoided by increasing the processing times, but this also increases costs. Dewaxing times of a week or more have sometimes been reported, but such long times are rarely if ever practical for commercial use.
Another practical difficulty well known in the art is the inconsistency of molding results often obtained from one lot of powder to another. Because the amount of internal surface is subject to variations from factors that are not always adequately controlled during the manufacture of practical powders, it is a common experience that a recipe for a molding dispersion optimized for one lot of powder of a certain type often will prove unsuitable for a subsequent lot supposedly manufactured under identical conditions. This variability is believed to result from variations in the amount of internal surface from one powder lot to another.
Although all these disadvantages of internal surface and volume in ceramic molding powders have long been known in the art, the practical methods of producing molding powders of most ceramic compositions almost always yield powders with substantial amounts of internal surface and volume, and heretofore few if any means for overcoming the adverse effects of such powder have been known in the art.
One practically important example of a dispersion suitable for injection molding is a dispersion of silicon nitride powder in a paraffin, polystyrene, and/or polyethylene binder. In the prior art it has been noted that the maximum powder loading possible for such a dispersion, without exceeding practical viscosity limits for injection molding, is about 80% by weight (hereinafter "w/o") when the average silicon nitride particle size and specific surface area (hereinafter "SSA") are about 1 micron and 12-15 square meters per gram (herinafter "m.sup.2 /g") respectively. Powder loadings in the range of 85-87 w/o have been achieved with larger sized powders with an SSA of about 8 m.sup.2 /g, but such powders do not sinter as well as those with SSA's of at least 12 m.sup.2 /g. It has been found impractical to achieve final product densities of more than 98% of the theoretical maximum for silicon nitride when using the larger sized silicon nitride powders with which loadings over 80 w/o have been previously achieved. For impervious final ceramic products, the higher possible density is generally preferred.
U.S. Pat. No. 4,207,226 to Storm describes the use of titanate coupling agents to reduce the viscosity and increase the practical loading levels of fine silicon carbide powders in organic binders. The coupling agents are merely mixed into the organic binder components. The Storm teaching is directed only to the processing of sinterable metal carbides. (Cf. Storm column 1, lines 11-13 and column 3, line 41.) Similarly, U.S. Pat. No. 4,056,588 to Baniel teaches the use of coupling agents for oxide and carbide ceramics.
Because of the particularity of surface effects in chemistry generally, it is not considered that teachings for the processing of carbides and/or oxides makes obvious the ability of the same materials to improve the processing of silicon nitride. This is especially true because it is believed that powder dispersion is controlled primarily by acid-base interactions among the powder, the dispersion medium, and any dispersing aids used, and because the surface of silicon nitride is generally basic while the surface of silicon carbide is generally acidic.